Both technologies for the production of methanol and for the production of ammonia use hydrogen as a main raw material and require technology to produce a synthesis gas containing hydrogen and carbon monoxide, and the production process of ammonia comprises sections derived from the technologies to produce a synthesis gas by reforming raw materials similar to those to produce a synthesis gas for methanol synthesis, and further includes a section to generate carbon dioxide and hydrogen by CO conversion of the synthesis gas formed by steam reforming, a section to remove carbon dioxide which is increased by an equal amount through the CO conversion, and a section to purify hydrogen to be suitable as a feedstock for ammonia synthesis by removing or inactivating the traces of remaining carbon monoxide and carbon dioxide. Thus, the primary reforming and/or the secondary reforming technologies of natural gas and the like can be utilized in common, and thereby various methods of the coproduction of methanol and ammonia have been conventionally proposed.
The raw materials required for producing methanol are hydrogen, carbon monoxide, and carbon dioxide, and these gases are the main components of a synthesis gas obtained by the steam reforming of a hydrocarbon, such as natural gas, LPG, butane or naphtha, and it is desired to have a ratio fulfilling the chemical stoichiometric formulas for methanol synthesis, which are:CO+2H2→CH3OH  (1)CO2+3H2→CH3OH+H2O  (2),
and it is ideal that the following R value, as the stoichiometrically-factored number,R=(mole number of H2−mole number of carbon dioxide)/(mole number of CO−mole number of carbon dioxide)  (3)is equal to 2, while the synthesis gas obtained from the steam reforming generally has the R value larger than 2 and is rich in hydrogen as compared to carbon.
In contrast, the raw materials required for producing ammonia are hydrogen and nitrogen, and it is important to adjust the flow ratio of hydrogen, which is a main component of the purified synthesis gas for ammonia synthesis, obtained by a purification method, in which carbon dioxide is removed from the synthesis gas obtained through CO conversion allocated after the steam reforming (primary reforming) and the oxygen reforming (secondary reforming) of a hydrocarbon, such as natural gas, LPG, butane or naphtha, in order to convert carbon monoxide to carbon dioxide for the essential removal of carbon dioxide by CO2 absorption (CO2 removal), and traces of remaining carbon monoxide and carbon dioxide are further separated at a low temperature or become inactive to ammonia synthesis catalyst by methanation, and nitrogen entrained by the purified synthesis gas, which is contained in the air used for the oxygen reforming (secondary reforming) and is an inert to the reforming reaction, is adjusted to the stoichiometric 3-to-1.
In conventional methods of coproducing methanol and ammonia, the following methods have been proposed of adjusting the flow ratio of hydrogen-to-nitrogen to 3-to-1.
JP-A 55-154314 (FIG. 1, lower right-handed column of page 2), describes a method of preparing synthesis gas for ammonia synthesis by adjusting the flow ratio of hydrogen and nitrogen in the purified synthesis gas, wherein the purge gas from a main methanol synthesis facility treated by the primary reforming and the secondary reforming prior to the high temperature CO conversion, the low temperature CO conversion, the CO2 removal, the methanation further processing the remaining carbon monoxide and carbon dioxide, for the purpose of obtaining the stoichiometric ratio of 3-to-1 as a prior art. It describes that a method of producing ammonia that consumes less energy, by way of preparing the synthesis gas for ammonia synthesis by omitting the process of low temperature CO conversion and by the process of removing traces of carbon monoxide and carbon dioxide, which are catalyst poisons for an ammonia synthesis catalyst, not by the methanation process (2.5 MPa), but by an auxiliary methanol synthesis facility at a high pressure (from 10 to 30 MPa) applying that carbon monoxide and carbon dioxide become feedstocks for methanol synthesis of the reaction formulas (1) and (2) respectively.
In addition, in JP-A 56-120514, DE-A1 3336649, and GB-A 2252317 (Claim 1), methods of ammonia synthesis are disclosed in which, since the purge gas from the methanol synthesis facility described above is rich in hydrogen and can be equivalent to the reformed gas of the primary reformer in a conventional ammonia production process, it is possible to adjust the flow ratio of hydrogen and nitrogen in the synthesis gas to the stoichiometric ratio of 3-to-1 and also possible to reduce the energy consumption even when omitting the primary reformer from the conventional ammonia production process and using the purge gas only in the secondary reformer.
In JP-A 2000-63115 ([0006] [0011]), although a method is disclosed of coproducing methanol and ammonia by omitting the CO conversion section and providing the methanol synthesis section after the CO2 removal section in a conventional method of producing synthesis gas for ammonia from natural gas, it is explicitly expressed that the amount of removing the carbonoxide components (carbon monoxide and carbon dioxide) is limited because carbon dioxide is removed without using a shift reactor which converts carbon monoxide into carbon dioxide. Thus, the method of coproducing methanol and ammonia is to be established in the case of a relatively high methanol production ratio and is generally applied to the case of producing a larger amount of methanol than ammonia in terms of weight ratio. In addition, it is almost impossible to produce only methanol or ammonia without remarkably decreasing the efficiency.
In JP-B2 7-33253, a method is disclosed of synthesizing a smaller amount of methanol than ammonia. It describes that in a conventional ammonia production process from natural gas, a compressed synthesis gas after methanation is drawn out from the synthesis gas in the middle stage of the compression section so as to make the ratio of hydrogen to nitrogen in the synthesis gas larger than 3, separating and extracting hydrogen for methanol synthesis by a membrane or the like, and mixing with carbon dioxide from the CO2 removal section to be supplied to the methanol synthesis section.
In WO 97/10194, a method is disclosed of synthesizing methanol by bypassing a portion of the synthesis gas from the secondary reformer, in a conventional ammonia production process from natural gas, over the high temperature CO conversion section to put into the methanol synthesis section in one pass. Although the insertion of such a methanol synthesis section here exhibits the performance as an alternative for the CO conversion reaction at a high temperature and, at the same time, gives the characteristic of being capable of producing not just carbon dioxide but more useful methanol, the conversion rate of carbon to methanol is remarkably low due to the presence of the chemical equilibrium of the CO shift reaction. Thus, it becomes clear that the amount of methanol is too small to be referred to as coproduction of methanol and ammonia, which does not go beyond the sense of the language of by-production. Moreover, it is almost impossible to produce only methanol without remarkably decreasing the efficiency.
A method is generally known in which hydrogen gas containing a small amount of methane is obtained by removing carbon dioxide after recovering methanol contained in the purge gas and then, nitrogen obtained from an air separation unit is added to it so as to make the ratio of hydrogen to nitrogen to be 3-to-1, which thereby prepares a feedstock gas for ammonia synthesis as another method of producing ammonia by using purge gas rich in unreacted hydrogen from methanol synthesis facility as hydrogen source of feedstock.
In contrast, in the case of using an air separation unit, a balancing amount of utilized nitrogen and that of oxygen to be separated is important. US-A1 2007/0299144 discloses a method of using high-purity oxygen from air separation as a reforming agent of an autothermal reformer for synthesis gas production, in which a method is proposed such that the autothermally reformed gas is divided into two streams and carbon monoxide in one of the streams is subjected to CO conversion and is then removed as carbon dioxide, and thus hydrogen is obtained as a feedstock for ammonia synthesis and a portion of the hydrogen is mixed with the rest of the autothermally reformed gas, and thereby producing synthesis gas for methanol having the R value so-adjusted to be optimum for methanol synthesis and calculated by the formula (3) described above. At the same time, it is further possible to produce a synthesis gas for ammonia having the ratio of hydrogen to nitrogen of 3-to-1 by adding nitrogen obtained by air separation to the residual amount of the hydrogen as the feedstock for ammonia synthesis and free of carbon dioxide by the CO2 removal, including the carbon dioxide converted from carbon monoxide through the CO conversion.